Workers in the thermoelectric industry have been attempting too improve performance of thermoelectric devices for the past 20–30 years with not much success. Most of the effort has been directed to reducing the lattice thermal conductivity (K) without adversely affecting the electrical conductivity. Experiments with super-lattice quantum well materials have been underway for several years. These materials were discussed in an paper by Gottfried H. Dohler which was published in the November 1983 issue of Scientific American. This article presents an excellent discussion of the theory of enhanced electric conduction in super-lattices. These super-lattices contain alternating conducting and barrier layers and create quantum wells that improve electrical conductivity. These super-lattice quantum well materials are crystals grown by depositing semiconductors in layers whose thicknesses is in the range of a few to up to about 100 angstroms. Thus, each layer is only a few atoms thick. (These quantum well materials are also discussed in articles by Hicks, et al and Harman published in Proceedings of 1992 1st National Thermoelectric Cooler Conference Center for Night Vision & Electro Optics, U.S. Army, Fort Belvoir, Va. The articles project theoretically very high ZT values as the layers are made progressively thinner.) The idea being that these materials might provide very great increases in electric conductivity without adversely affecting Seebeck coefficient or the thermal conductivity. Harmon of Lincoln Labs, operated by MIT has claimed to have produced a super-lattice of layers of (Bi,Sb) and Pb(Te,Se). He claims that his preliminary measurements suggest ZTs of 3 to 4. FIG. 1 shows theoretical calculated values (Sun et al—1998) of ZT plotted as a function of quantum well width.
The present inventors have demonstrated that high ZT values can definitely be achieved with Si/Si0.8Ge0.2 super-lattice quantum well. (See, for example, U.S. Pat. No. 5,550,387.) The present inventors have had issued to them United States patents in 1995 and 1996 which disclose such materials and explain how to make them. These patents (which are hereby incorporated by reference herein) are U.S. Pat. Nos. 5,436,467, 5,550,387. FIGS. 1A and 1B herein were FIGS. 3 and 5 of the '467 patent. A large number of very thin layers (in the '467 patent, about 250,000 layers) together produce a thermoelectric leg 10 about 0.254 cm thick. In the embodiment shown in the figures all the legs are connected electrically in series and otherwise are insulated from each other in an egg-crate type thermoelectric element as shown in FIG. 1A. As shown in FIG. 1B current flows from the cold side to the hot side through P legs and from the hot side to the cold side through N legs. (Electrons flow in the opposite direction.) These patents disclose super-lattice layers comprised of: (1) SiGe as conducting layer and Si as a barrier layer and (2) alternating layers of two different alloys of boron carbide. In the '387 patent Applicants disclose that they had discovered that strain in the layers can have very beneficial effects on thermoelectric properties of the elements disclosed in the '467 patent.
U.S. Pat. Nos. 6,096,964 and 6,096,965 disclose techniques for making thermoelectric elements by depositing thin alternating layers of semi-conductor material and barrier layers on very thin substrates to create quantum wells. All of the above-cited patents are incorporated herein by reference.
What are needed are improved thermoelectric devices that can be produced efficiently for practical application.